Recently, in the field of semiconductor technologies, integrated circuit boards (IC) have been demanded to become a highly integrated design and a widely versatile design. Along with this tendency, IC packages for placing an IC chip thereon have also been remarkably rapidly enhanced in their performance such as, for example, small design, increased capacity, thin design, increased number of pins, and the like. As the performance has been enhanced, their heating value also tended to be consistently increased. For example, IC packages having a heating value of several W have been increasingly provided with a heat dissipating plate called "heat spreader". IC packages having such a large heating value as exceeding 10 W have been increasingly provided with a heat dissipating plate called "heat sink".
For example, the heat spreader is typically a regular square sheet having a side length dimension of about 25 to 45 mm and a thickness of about 0.5 to 1.0 mm and has a recess formed in the center of its single surface which recess is intended for an IC to be placed therein. Such a heat spreader is mounted on a substrate with an IC placed thereon and molded with resin so as to form an IC package. In that state of the IC package, the surface of the heat spreader is exposed outside to dissipate heat. Some heat spreaders are formed on their upper surface or lower surface with fins which are adapted to enhance the effect of heat dissipation or heat absorption. Fins are sometimes formed on a lower surface of each heat spreader which lower surface is embedded in resin. A plurality of holes for filling the resin therein are also occasionally formed in the lower surface of the heat spreader, so that the heat spreader will not peel off the resin. Conventionally, such heat spreaders are produced by plastic working, for example, punching, drawing, rolling a metal sheet with high thermal conductivity, such as copper, aluminum, or the like. The aforementioned heat sink is, in use, placed on the IC package on which the heat spreader is placed, in order to further enhance the heat dissipation of the heat spreader. Such heat sinks are also produced likewise by plastic working, for example, bending, extruding, punching a metal sheet with high thermal conductivity, such as aluminum, or the like. In particular, in the case that the fins are comparatively large in height, they are conventionally formed by engraving through machining.
As previously discussed, IC packages have been tended to be consistently increased in heating value. Along with this tendency, more enhanced effect of heat dissipation for heat spreaders and heat sinks has naturally been increasingly demanded. One approach was made for this purpose to increase the number and length dimension of the fins. However, it is difficult to increase the number and length dimension of the fins by the plastic working technique. If machining is employed in order to obviate this shortcoming, another shortcoming is encountered in that the cost is increased. Additionally, there is still another shortcoming in that the holes formed in the heat spreader for the purpose of preventing the heat spreader from peeling off resin is not good enough when a large force is applied or in other similar cases.
With plastic working, distortion created during plastic working occasionally inevitably remains in final products in the form of residual stress. If such residual stress is left as it is, the heat spreader would be warped or flexed because the residual stress is released by heat when a circuit is formed on a substrate on which the heat spreader is placed, by bonding, or the like. Such deformation has the risk to degrade the accuracy of flatness of the surface of the substrate on which the heat spreader is to be mounted on the substrate, thereby not only degrading the intimacy between the heat spreader and the substrate but also resulting in many other disadvantageous effects for the IC package as a whole, such as, breakage of a connecting wire, peeling-off of an IC chip, poor formation of solder balls, and the like. In order to avoid such inconveniences and disadvantages, a proposal was made, in which a sheet having a through hole formed by punching is attached to a central region of a heat spreader by an adhesive agent. However, since there is a need of two members for this proposed method, additional problems are newly created in that the number of processes is increased, the sheet tends to peel off the heat spreader during heating due to difference in thermal expansion between the adhesive agent and the sheet, or the rigidity becomes insufficient due to the fact that the sheet is attached to the heat spreader by bonding.
It is an object of the present invention to provide a method of manufacturing a sheet member in which amount of distortion is restricted and the accuracy of flatness is maintained at a high level, and which is especially suited to be used for manufacturing a heat dissipating plate such as a heat spreader, a heat sink, or the like, with or without fins which heat dissipating plate is resin molded, for example, to an IC package; and to provide a heat dissipating plate.